1. Field of the Invention
This invention relates to improved rotating elements. In particular, this invention relates to composite tubular elements for transmitting forces, and for sustaining axial and torque bearing forces.
2. Prior Art
Conventional rotating elements intended for transmission of forces such as rotor or drive shafts are generally made of metal, since these metal rotors or drive shafts are believed generally to possess great durability. As is well known, metal rotors or drive shafts, however, suffer from a number of disadvantages. For instance, it is impractical, if not impossible, to employ a single long metal drive shaft on a truck since, as the shaft is rotated, centrifugal forces act on the shaft mass. Consequently, any asymmetry in the shaft increases dramatically with an increase in the speed at which the shaft is rotated. The increased asymmetry causes the shaft to bend. Bending, however, is opposed by the elastic properties of the shaft metal, thereby resulting in a harmonic oscillation or vibration. The speed at which the amplitude of vibration is greatest, sometimes disastrously so, is referred to as the critical speed. For a long metal shaft for a truck, the critical speed is far too low for practical use.
In order to overcome the critical speed limitations of single long shafts, multiple sections of shafts are typically employed. Indeed, in the case of truck drive shafts, it is known to use up to four relatively short length solid metal cylinders in the transmission chain, one connected to the other by means of universal joints and the like rather than a single length of rotor shaft. At each joint, bearings are required, as well as mounting brackets and the like. These multiple components not only increase the overall weight of the truck, but more importantly they tend to wear in use completely offsetting the great durability normally associated with metal rotational shafts.
Thus, the permissive circumferential speed of a rotor shaft is determined by its design and by the material employed in its construction. The design of a rotor or drive shaft of lighter weight and with greater axial stiffness would permit, of course, the application of such a shaft in higher critical speeds than presently possible with all metal shafts as presently constructed. In the past, some attempts have been made to design a lighter drive shaft. For example, it is known to reinforce metal tubes with helically wound filaments which are subsequently impregnated with a resin such as an epoxy resin, thereby forming a composite structure which has a metal portion and a plastic portion reinforced with continuous filament windings. Such composite structures, while capable of withstanding very high circumferential speeds, suffer from numerous disadvantages. For example, such helically wound rotors have inadequate axial stiffness for drive shaft applications.
Another difficulty associated with fiber reinforced resin coatings on tubular metal shafts is associated with the significant difference in the physical properties of the two essential materials, i.e. the metal and the fiber reinforced plastic. To get the requisite performance from the rotor or drive shaft, both materials must be combined in such a way as to operate harmoniously in absorbing and transmitting substantial torsion, tension and compression loads. Also, it is worth noting that durability tends to be a problem when bonding two dissimilar materials, such as plastic to metal. Consequently, there still remains a need for an improved rotor or drive shaft that will have the necessary strength and weight and load carrying ability and which can be economically prepared.